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Cruise ships carrying more than 500 passengers will be prohibited from landing anyone. Only 100 visitors are to be allowed on shore at any given time, in an attempt to prevent damage to the region's unique ecosystem.

So it seems that the limits are for the ships to not allow more than 100 passengers on land in Antarctica. According to the same article the visitors to Antarctica are around 45,000 annually. With simple calculations you will find out that if these visitors stayed one day only there could be more than 100 people at the same time.